1. Field of the Invention
The present invention relates to a sheet stacking apparatus, a sheet processing apparatus and an image forming apparatus, and the present invention is preferably applied to a sheet processing apparatus which is stacked with sheets output from an image forming apparatus such as a copying machine, a laser beam printer and a processing apparatus or an image forming apparatus equipped with the same sheet processing apparatus.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 10-305961 has disclosed a sheet post-processing unit as a sheet stacking apparatus. This sheet post-processing unit has two or more stacking trays and a function of lifting up/down a stacking tray when sheets stacked in that stacking tray reaches a maximum loading capacity of the tray and changing over the stacking tray for stacking following sheets.
A variety of devices for increasing a loading capacity in the sheet stacking apparatus having a plurality of trays have been known. More specifically, according to Japanese Patent Application Laid-Open No. 2001-72325, when it is detected that a tray to which sheets are discharged currently becomes full in a stacking apparatus having a plurality of bin trays, a tray unit located above the tray which is currently receiving discharged sheets is lifted upward so as to increase the loading capacity of the tray which is currently receiving the discharged sheets.
However, the related art described in the Japanese Patent Application Laid-Open No. 2001-72325 has such a problem that although the loading capacity of the tray which is currently receiving discharged sheets can be increased, the loading capacity of the stacking tray located above is decreased by the same amount as an increase of the loading capacity.